Both investors want exclusive first look, and I have one pipeline

The rationing isn't the part I'd worry about. You're selling attention order, which is what any lead vendor does. What would bother me is A's 72 hours running while a seller is sitting there thinking someone is coming. If A passes on hour 71 and B calls on day four, that seller has been warm for three days with nobody on the phone. You lose leads that way, not just fees.

Exclusivity windows on raw lead information are ordinary lead-vendor practice and don't by themselves move you toward brokerage. The line that matters in most states is whether you're representing anyone's interest in a transaction or negotiating terms, and how that line is drawn varies state to state, so confirm it with a real estate attorney licensed where you're operating before you paper anything.

The economics are the weaker part of what you've described. At $25 per qualified lead you've created a definition problem. Who decides qualified? If B decides after seeing the address, you've just given him the lead for free on every one he declines. Write the qualification test before you send anything else, something mechanical like owner contacted, motivation stated, price expectation stated. If B won't agree to a mechanical test, that $180 a month is at his discretion and you shouldn't be budgeting around it.

On the tiering, your real exposure is that A's 72 hour window burns your relationship with the actual seller. A code violation notice recipient who talks to you and then hears from nobody for three days remembers that. You're the name they associate with the wasted call. Cut the window to 24 hours or make it start on A's acknowledgment rather than your send.

Also think about what happens when A closes on something he says he found independently. You have 30 to 40 names a month with no contract and no recording trail proving you sourced them. Keep a timestamped send log with addresses, and get the fee arrangement in writing including how disputes over sourcing get settled. A flat $1,000 on close is easy to forget when nothing binds it.

The qualified-lead definition thing is what stopped me cold when I looked at this. I asked one investor what qualified meant and he said "one I'd want to talk to," which is not a definition, it's a mood. If I were you I'd take B's $500 flat and drop the per-lead entirely rather than fight over 25 dollars a name.